Industrial drive mechanisms powered by compressed air possess the advantages of being fast-acting, clean and relatively inexpensive to install, operate and maintain. Most industrial plants have existing compressed air sources to which the mechanisms may be readily connected. Minor leakages of compressed air, and exhausting of it into the atmosphere, are usually permissible. The major disadvantage of purely pneumatic drive mechanisms is the fact that their output motions cannot be precisely controlled due to the inherent "springyness" of air, i.e., its capability for large magnitude compression and subsequent expansion.
The output motions of active hydraulic drive mechanisms can be more precisely controlled due to the relative incompressibility of hydraulic fluid in relation to air. However, the pumps and pump motors of active hydraulic drive mechanisms are noisy, expensive from both the acquisition and maintenance viewpoints, and the circuitry for conducting hydraulic fluid to and from such pump and its associated hydraulic fluid to and from such pump and its associated components necessarily includes various fittings potentially capable of hydraulic-fluid leakage.
In recognition of the aforesaid relative advantages and disadvantages of purely pneumatic and purely hydraulic drive mechanisms, the use of hybrid hydropneumatic drive apparatuses has heretofore been proposed. Such apparatuses employ an active pneumatic mechanism and a passive (i.e., "pumpless") hydraulic mechanism connected with each other and with a driven member in such a manner that movement imparted to the driven member by the pneumatic drive mechanism produces, and is controllable by restriction of, flow of fluid within the hydraulic mechanism. In all known prior hydropneumatic drive apparatuses other than those employing valves that are capable of manual adjustment only, the means employed for restricting the flow of fluid within the hydraulic mechanism consists of one or more valves operated by a solenoid or the like and having only two operating conditions, in one of which the fluid flow is either substantially unrestricted or substantially completely restricted, and in the other of which fluid flow is partially restricted to a constant extent. When a single such valve is employed, the only control achievable thereby over the motion of the driven member is to either abruptly halt such motion and/or to abruptly change the speed of the driven member between a maximum-attainable velocity and one other velocity of a lesser constant magnitude. In some of the prior-art hydropneumatic drive apparatuses, of which those disclosed in U.S. Pat. Nos. 2,878,873 and 3,802,318 are illustrative, a plurality of hydraulic fluid control valves of the aforesaid two-condition type are employed in association with each other. Selective actuation of the valves permits the speed of the driven member to be changed either directly or in discrete steps between its maximum attainable velocity and two or three, instead of just one, reduced velocities of predetermined constant magnitudes. While constituting an improvement over apparatuses employing a single two-condition valve for hydraulic fluid control, the multiple-valve apparatuses are similarly capable of effecting only limited variations in the velocity of the driven member, and are totally incapable of controlling the rate of the changes of the velocity of the driven member from one discrete level or magnitude to the next discrete level. The full performance capabilities of more sophisticated types of industrial robots and the like cannot be realized by a hydropneumatic drive apparatus which is capable of varying the speed of the driven robotic member between only a small number of discrete magnitudes or levels and which is incapable of controlling the positive and/or negative acceleration forces imposed upon the driven member during changes in its speed from one such level to the next.
Another significant deficiency in hydropneumatic drive apparatuses employing solenoid-operated valves for restricting the flow of hydraulic fluid is the slowness of operation of such valves. Each transition from one to the other of the valve's operating conditions must be preceded by the establishment or decay of a magnetic field of considerable intensity. The time delay required for such field to build and decay is significant when compared with the high speeds of operation and control desired in some utilizations of industrial manipulators or robots, and would detrimentally affect performance in such utilizations.
The tasks which an industrial robot or the like can be called upon to perform are quite varied, and at least some may require positioning of its driven member within tolerances of a few thousandths of an inch. In order to achieve such precise positioning of the driven member with a hydropneumatic drive apparatus, the latter must firstly include means capable of accurately identifying the position occupied by the driven member at substantially any possible location thereof along its path of travel, as opposed to only those positions happening by chance to exactly coincide with one of a plurality of spaced indicia of an associated measuring device. Secondly, the valve means restricting the flow of hydraulic fluid within the hydraulic mechanism of the apparatus must be capable of achieving quite minute changes in the fluid flow conditions when commanded to do so.